A modem is disclosed in copending application of O. Leon Pierce, Mark C. Smith, and R. Byron Driver, entitled Line Powered Modem, Ser. No. 203,108, now U.S. Pat. No. 4,395,590, filed contemporaneously herewith, and assigned to the same assignee as this invention, in which the entire operating power for the modem circuits is derived from the available telephone line power. This modem includes an RS232 output interface circuit which provides a carrier detect signal and a received data signal which must be within predetermined voltage ranges to be properly received by the associated business machine or other utilization apparatus to which the modem is connected. This output interface circuit must be appropriately powered to provide the intended output signal levels.
In the aforesaid line-powered modem, electro-optical isolators are provided to isolate the telephone line from the utilization equipment. Such isolators include an LED which provides light signals in response to applied electrical signals, and a photodetector providing a corresponding electrical output signal in response to received light signals from the LED. Electro-optical isolators are generally known for providing isolation between input and output circuits. However, isolators of known construction have a relatively slow speed of response to low current signals. As a result, current levels must be sufficiently high to provide adequate response speed. The utility of such conventional isolators in a line-powered modem is materially impaired, since in line-powered modems, only low current levels are available and an increased speed of response cannot be achieved by increasing the current drive to the isolator LED.
The relatively slow response time of the isolator is caused by presence of stored charge in the phototransistor which must be allowed to bleed off in order to turn the transistor off. If the only path for dissipation is through the high-impedance base-emitter junction of the phototransistor, the process can take hundreds of microseconds before sufficient charge is dissipated to turn the transistor off. Moreover, the Miller effect capacitance, which is the collector-base capacitance of the phototransistor, causes, in response to large changes in voltage at the collector, a current to flow through that spurious capacitance to the base which opposes the turn-off operation.